1. Field of the Invention
The present invention relates to an apparatus for generating ozone; and, more particularly, to an apparatus for generating ozone in high concentration with high efficiency and controlling the density of the ozone being generated.
2. Description of the Related Art
Ozone (O3) has been used for various applications because of its strong oxidization, decolorization, deodorization, and sterilization properties. It is, however, impossible to store ozone under room temperature and atmospheric pressure since it tends to reduce to oxygen (O2). Thus, in order to use ozone, one has to use an ozone generating apparatus, which uses oxygen or air as a raw material for ozone.
Several methods have been developed for ozone generation, such as a method using ultraviolet rays, a silent discharge method and a method of electrolyzing water, etc. Among these methods, a silent discharge method using high-voltage has been used for a wide variety of industrial applications because it can efficiently produce highly concentrated ozone (See, Siemens W. 1857, Ann. Phy. Chem. 102, 66-122).
As is well known in the art, the silent discharge method uses two metal electrodes with one or both of them insulated with a dielectric material. An AC (alternating current) signal is applied to the electrodes and a discharge is then performed in a space between the metal electrodes while oxygen-containing air passes through the space, thereby transforming some of the oxygen into ozone.
An ozone generator using the silent discharge method can be implemented using various shapes and structures. The most popular structure is a cylindrical structure that employs one or more glass pipes. In this method, each of the glass pipes is installed inside metal cylinders being used as ground electrodes, and the glass pipes are coated on the inside with a metal film to provide a high-voltage electrode. This type of cylindrical ozone generator, however, is too voluminous for practical use, and it does not provide a uniform discharge because of the difficulty of maintaining constant intervals between the glass pipes and metal cylinders. Moreover, the glass pipe used as a dielectric become corroded by ozone generated therein, which causes the dielectric breakdown. To overcome such limitations in the conventional cylindrical ozone generators and to raise concentration of ozone being generated, U.S. Pat. No. 5,759,497 discloses a flat plate type ozone generator using flat plate type ceramic as a dielectric, sometimes referred to as an “Otto-Plate type ozone generator.”
To perform a silent discharge by using various types of ozone generators as discussed above, a high-voltage sine wave signal with a commonly used frequency, e.g., 60 Hz (Hertz), is adopted. It is, however, difficult to raise ozone concentration by using an AC signal at such a frequency. Therefore, an ozone generator using an inverter to generate a mid-range frequency signal of about 1 kHz (kilo-Hertz) has recently been developed.
Nevertheless, it is still difficult to effectively raise the ozone concentration because the high-voltage signal is a sine wave.
There are several methods for controlling the ozone concentration in ozone generating apparatuses, such as changing the voltage level or changing the frequency of the high-voltage signal used in ozone generation, and changing the pulse width. However, owing to the characteristics of the silent discharge, it is considerably difficult to linearly control the ozone concentration by these methods. Specifically, when changing the voltage level of a high-voltage signal applied to an ozone generating apparatus, the ozone concentration is normally increased as the voltage level is increased. However, the relationship between the concentration of ozone generated by the ozone generating apparatus and the voltage level applied to the apparatus is not linear, and the silent discharge can be performed only when the voltage level is equal or above a predetermined level. Therefore, it is very difficult to linearly control the ozone concentration by changing the voltage level of the high-voltage signal.
In instances where the frequency of the high-voltage pulse is changed to control the ozone concentration, optimal efficiency in ozone generation cannot be obtained because of impedances between the ozone generator, the high-frequency inverter, and the high-voltage transformer cannot be matched due to the frequency change. Additionally, in instances where the pulse width of the high-voltage pulse is changed, the ozone concentration stops increasing when the pulse width increases over an optimal pulse width.